584 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS
TABLE 11
Typical electrogenerated titrants and substances determined by coulometric titration*Electrogenerated
TitrantGenerating Electrode
and SolutionTypical Substances DeterminedBromine Pt/NaBr As(III), U(IV), NH 3 , olefins,
phenols, SO 2 , H 2 S, Fe(II)
Iodine Pt/KI H 2 S, SO 2 , As(III), water (Karl
Fischer), Sb(III)
Chlorine Pt/NaCl As(III), Fe(II), various organics
Cerium(IV) Pt/Ce 2 (SO 4 ) 3 U(IV), Fe(II), Ti(III), I
Manganese(III) Pt/MnSO 4 Fe(II), H 2 O 2 , Sb(III)
Silver(II) Pt/AgNO 3 Ce(III), V(IV), H 2 C 2 O 4
Iron(II) Pt/Fe 2 (SO 4 ) 3 Mn(III), Cr(VI), V(V), Ce(IV),
U(VI), Mo(VI)
Titanium(III) Pt/TiCl 4 Fe(III), V(V, VI), U(VI), Re(VIII),
Ru(IV), Mo(VI)
Tin(II) Au/SnBr 4 (NaBr) I 2 , Br 2 , Pt(IV), Se(IV)
Copper(I) Pt/Cu(II)(HCl) Fe(III), Ir(IV), Au(III), Cr(VI), IO 3
Uranium(V), (IV) Pt/UO 2 SO 4 Cr(VI), Fe(III)
Chromium(II) Hg/CrCl 3 (CaCl 2 )O 2 , Cu(II)
Silver(I) Ag/HclO 4 Halide ions, S^2 , mercaptans
Mercury(I) Hg/NaClO 4 Halide ions, xanthate
EDTA Hg/HgNH 3 Y^2 ^ a Metal ions
Cynaide Pt/Ag(CN) 2 Ni(II), Au(III, I), Ag(I)
Hydroxide ion Pt()/Na 2 SO 4 Acids, CO 2
Hydrogen ion Pt()/Na 2 So 4 Bases, CO 32 , NH 3
a Y 4 is ethylenediamine-tetra-acetate anion.
* From A.J. Bard and L.R. Faulkner, Electrochemical Methods, p. 390. (Copyright 1980 by
John Wiley & Sons, Inc., with permission.)(1) Detectors
Radiation detectors operate on two principles; ionization
of a gas or solid to generate a small ion current and excita-
tion of a substance to cause a short term luminescence in a
crystal or a solution. Ionization detectors are of two varieties;
those that use gas as the ionization medium, i.e., gas-filled
detectors and those that use a crystal, i.e., semiconductor
detectors. Measurements of luminescence due to radiation
are made in crystal and liquid scintillation counters.(a) Ionization and detectors
(i) Gas-filled detectors
Gas-filled detectors respond to ionizing radiation by the
formation of ion pairs, a photoelectron and a positive ion
(cation) by the gas molecules. The gas in the detector tube may
be a mixture: e.g., argon and a low concentration of an organic
substance or methane. In Figure 38 the gas-filled detector is
shown with a central electrode (anode) to which is applied a
voltage. The pulse of photoelectrons migrate to the electrode
due to the electric field, are collected, and produce a small ion
current. The current represents the radiation event or particlesdeposited in the detector. Several types of gas-filled detectors
are available and are operated in a number of voltage ranges
leading to different kinds of detector responses; the operation
potentials of these detectors are illustrated in Figure 39.
In the saturation (ionization) chamber region a radia-
tion event of given energy gives rise to a number of pho-
toelectrons independent of the applied potential. They are
collected on the electrode yielding an ion current or pulse
height proportional to the energy of the radiation event. The
pulse of photoelectrons, however, has a pulse width due to
the random nature of the collision process. This situation
leads to a statistical variation in pulse heights for the same
energy source defining the resolution of the detector. An ion-
ization chamber in combination with a pulse height analyzer
can operate as an alpha particle spectrometer.
When the applied voltage is increased, producing a field
greater than 200 V/cm, the electrons formed in ion pair for-
mation are accelerated. These accelerated electrons collide
with gas molecules causing increased ionization leading to
an increase in the collected electrons. This effect is called
gas amplification and is present in the proportional counter
and GM tube regions.C009_005_r03.indd 584C009_005_r03.indd 584 11/23/2005 11:12:29 AM11/23/2005 11:12:29